This research effort is designed to separate, isolate and define information processing stages related to the evaluation of a stimulus, and to determine which stages are sensitive to cholinergics and in what manner. An equally important goal is to determine which stages of information processing are affected by aging, and whether these are the same stages that are cholinergic-sensitive. To this end three experimental tasks are proposed which systematically separate several features of stimulus processing. Old people may have difficulties in early visual processing, decision making, memory and attention, along with slowing of response time. The use of an information processing approach based on a serial model allows a dissection of the reaction process and the isolation of particular problem areas. Recent work shows drugs related to the different neurotransmitter systems have specific effects on the different information processing stages, and aging may involve changes in these neurotransmitter systems. The design of our study will allow us to integrate three areas of research that, in the past, have seldom overlapped. The first area concerns the changes in specific neurotransmitter systems that occur with aging and psychopathology. There have been many indications that in senile dementia of the Alzheimer's type (SDAT) the cholinergic system is involved. Less dramatic declines may be associated with normal aging and perhaps with other dementias, including schizophrenia. The second area concerns the hypothesis that at least a major part of the cognitive deficit in aging depends on changes in stimulus processing rather than response choice, decision making or purely motoric slowing. The third area is very recent work suggesting that stimulus evaluation depends on a cholinergic arousal system. At the theoretical level the research proposed here integrates cognitive and pharmacologic models of the aging process by the isolation of specifically impaired information processing stages and relating this impairment to the neurochemical changes that occur in aging. At the practical level, once the sensitivity of a given task to the cholinergic neurotransmitter system is established, it will provide a noninvasive method of assessing the functional states of that system.